With the recent rapid advances in technologies such as lasers, electro-optics, fiber optics and other similar photometric sciences, there has grown a requirement for high-accuracy, fast yet simple and inexpensive optical power measurement and calibration techniques.
A currently available optical power measurement device known as Electrically Calibrated Pyroelectric Radiometer (ECPR) is practical and highly accurate. It has a + or xe2x88x921% uncertainty rate and, unlike most other optical power measurement devices, is wavelength-independent over a broad spectral range. Further, it is designed to respond to power change. Therefore, incoming continuous wave optical power, to be incident on the ECPR, must be chopped or AC-modulated when making optical power measurements. The National Institute of Standards and Technology (NIST) uses the ECPR as its optical power transfer calibration standard.
The substitution method is employed by many calibration techniques. It consists of comparing the unit under test (UUT) with the chosen standard. The UUT is said to be transfer-calibrated by the standard. Naturally, each transfer-calibration that is made further down the measurement process from the standard adds uncertainty. Therefore, it is desirable to reduce the number of intermediate steps in the measurement chain. One way to achieve such a reduction is to use a higher-level standard, rather than some other intermediate standard, at the location where the UUT is to be calibrated.
Many laboratories which need to calibrate their UUT""s already have fiber-coupled optical power sources at the several wavelengths required to perform optical power transfer-calibration by the substitution method. At present, these laboratories typically use an intermediate standard to transfer-calibrate the UUT. What is needed is a low-cost and easy-to-use means to utilize ECPR or other higher-level standard in conjunction with fiber-coupled optical power sources, optical attenuation and other equipment already existing in these laboratories to perform the necessary calibration.
By employing a high-repeatability optical switch 11 that can transmit input optical power selectively either to the standard or the unit under test (UUT), OPHASE presents a means for performing a rapid, repeatable comparison between the standard and the UUT, thereby eliminating the requirement for expensive optical power monitoring/compensation systems or reference detectors to obtain optical power stability. Further, the selective outing of beam traveling through one of the output fibers that are coupled to the witch either to the standard or the UUT eliminates much of the system uncertainty by enabling initial (i.e. prior to testing any UUT) characterization of the inequivalence, Im, and the ratio, Rp, between the power outputs of the multiple output fibers coupled to the switch. This characterization is accomplished by using angled interface 45 which allows simultaneous coupling of the multiple output fibers to the angled interface so as to enable the power readout of all the output fibers at the standard. Rp and Im are then used to calculate the correction factor that reduces the total uncertainty level in the subsequent measurement of the UUT.